Instant Notes: Analytical Chemistry

fashion to the other standards but omitting the analyte). The data is either

plotted as a calibration graphor used to calculate a factorto convert detector

responses measured for the analyte in samples into corresponding masses or

concentrations (Topic B4).

● Standard addition.

● Internal standardization.

The last two methods of calibration are described in Topic B4.

Instruments and apparatus used for analytical work must be correctly main-

tained and calibrated against reference values to ensure that measurements are

accurate and reliable. Performance should be checked regularly and records kept

so that any deterioration can be quickly detected and remedied. Microcomputer

and microprocessor controlled instrumentation often has built-in performance

checks that are automatically initiated each time an instrument is turned on.

Some examples of instrument or apparatus calibration are

● manual calibration of an electronic balance with certified weights;

● calibration of volumetric glassware by weighing volumes of pure water;

● calibration of the wavelength and absorbance scales of spectrophotometers

with certified emission or absorption characteristics;

● calibration of temperature scales and electrical voltage or current readouts

with certified measurement equipment.

Materials or substances suitable for use as chemical standards are generally

single compounds or elements. They must be of known composition, and high

purity and stability. Many are available commercially under the name AnalaR.

Primary standards, which are used principally in titrimetry (Section C) to

standardize a reagent (titrant) (i.e. to establish its exact concentration) must be

internationally recognized and should fulfil the following requirements:

● be easy to obtain and preserve in a high state of purity and of known chemical

composition;

● be non-hygroscopic and stable in air allowing accurate weighing;

● have impurities not normally exceeding 0.02% by weight;

● be readily soluble in water or another suitable solvent;

● react rapidly with an analyte in solution;

● other than pure elements, to have a high relative molar mass to minimize

weighing errors.

Primary standards are used directly in titrimetric methods or to standardize

solutions of secondaryor working standards (i.e. materials or substances that do

not fulfill all of the above criteria, that are to be used subsequently as the titrant in

a particular method). Chemical standards are also used as reagents to effect

reactions with analytes before completing the analysis by techniques other than

titrimetry.

Some approved primary standards for titrimetric analysis are given in Table 1.

Reference materials are used to demonstrate the accuracy, reliability and com-

parability of analytical results. A certified or standard reference material(CRM

or SRM) is a reference material, the values of one or more properties of which

have been certified by a technically valid procedure and accompanied by a trace-

able certificate or other documentation issued by a certifying body such as the

Reference
material

Chemical
standard

16 Section A – The nature and scope of analytical chemistry